Influence of the starting microstructure of an advanced high strength steel on the characteristics of Zn-Assisted liquid metal embrittlement

摘要

High temperature tension tests were performed to investigate the influence of variations in the starting micro-structure on zinc (Zn)-assisted liquid metal embrittlement (LME) susceptibility of an advanced high strength C-Mn-Si steel. Using a single alloy, the starting microstructure was modified by heat-treatment prior to zinc electroplating. The hot tension test data revealed comparable Zn-LME susceptibility for different martensite and bainitic-ferrite based microstructures generated through full austenitization, such as as-quenched martensitic, quenched and partitioned (Q&P), and transformation-induced-plasticity (TRIP)-assisted bainitic ferrite (TBF). LME cracks propagated through these microstructures via intergranular fracture along prior austenite grain boundaries, with no apparent involvement of microstructural components like retained austenite or carbide-free bainite in the LME cracking behavior. In contrast, a dual phase (DP) microstructure variant of the same steel, generated through intercritical annealing (partial austenitization), exhibited suppressed LME, revealing that the starting microstructure can influence Zn-LME susceptibility. The lower LME susceptibility of the DP steel compared to the martensitic, Q&P, or TBF steels is explained by the presence of ultraflne ferrite grains and discrete martensite islands, and a smaller area fraction of prior austenite grain boundaries in the DP microstructure.